North South Trending Rifts Research Articles

Seismic structure of the 2015 Mw7.8 Gorkha earthquake revealed by ambient seismic noise and teleseismic surface wave tomography

The destructive 2015 Mw7.8 Gorkha earthquake occurred in the Main Himalayan Thrust due to the collision of the Indian and Asian plates, which provides a unique opportunity to understand the deep dynamic processes and seismogenic mechanisms of strong earthquakes. We construct a regional-scale shear-wave velocity model of the crust and uppermost mantle using ambient seismic noise and teleseismic surface wave at periods of 5–100 s around the Gorkha earthquake region. The new shear-wave velocity model exhibits prominently lateral heterogeneities in the Gorkha earthquake areas. We observe a high-velocity (high-V) zone around the Gorkha main shock in the Main Himalayan Thrust, indicating the existence of a high-strength asperity that sustains the stress accumulating. The aftershocks are primarily located in the low-velocity (low-V) anomalies and enclosed by two high-V anomalies, which appear to act as structural barriers that influence the spread of the aftershocks. Prominent low-Vanomalies from the lower crust to the mantle lithosphere are observed along the north–south trending rifts, suggesting the hot materials upwelling due to the tearing of the northward subducting Indian lithosphere. These observations may indicate that seismic velocity heterogeneity could play an essential role in earthquake initiation and the rupture process.

Assessing the Activity of Eastern Himalayan Extensional Structures: Evidence from Low-Temperature Thermochronology of Granitic Rocks from Yadong

The east–west-trending South Tibetan Detachment System (STDS) and north–south-trending rifts (NSTRs) are the two main types of extensional structures that have developed within the Tibetan Plateau during continent–continent collision since the early Cenozoic. They have played significant roles in the evolution of the plateau, but it is unclear how they are related genetically. In the Yadong area of the eastern Himalaya, the NSTRs cross-cut the STDS. Apatite and zircon fission track ages of a leucogranite pluton in the footwall of the two extensional faults can be used to reconstruct the cooling and exhumation history and thereby constrain the activity of extensional structures. The new AFT ages range from 10.96 ± 0.70 to 5.68 ± 0.37 Ma, and the ZFT age is 13.57 ± 0.61 Ma. Track length distributions are unimodal, albeit negatively skewed, with standard deviations between 1.4 and 2.1 µm and mean track lengths between 11.6 and 13.4 µm. In conjunction with previously published datasets, the thermal history of the region is best explained by three distinct pulses of exhumation in the last 16 Ma. The first pulse (16–12 Ma) records a brittle slip on the STDS. The two subsequent pulses are attributed to the movement on the Yadong normal fault. The normal fault initiated at ~12 Ma and experienced a pulse of accelerated exhumation between 6.2 and 4.7 Ma, probably reflecting the occurrence of two distinct phases of fault activity within the NSTRs, which were primarily instigated by slab tear of the subducting Indian plate.

Upper Mantle Velocity Structure Beneath the Yarlung–Tsangpo Suture Revealed by Teleseismic P-Wave Tomography

We applied teleseismic tomography to investigate the 3D P-wave velocity (Vp) structure of the crust and upper mantle at depths of 50–400 km beneath the Yarlung–Tsangpo suture (YTS), by using 6164 P-wave relative travel-time residuals collected from 495 teleseismic events recorded at 20 three-component broadband seismograms. A modified multi-channel cross-correlation method was adopted to automatically calculate the relative arrival-time residuals of all teleseismic events, which significantly improved the efficiency and precision of the arrival-time data collection. Our results show that alternating low- and high-Vp anomalies are visible beneath the Himalayan and Lhasa blocks across the YTS, indicating that strong lateral heterogeneities exist beneath the study region. A significant high-Vp zone is visible beneath the southern edge of the Lhasa block at 50–100 km depths close to the YTS, which might indicate the rigid Tibetan lithosphere basement. There exists a prominent low-Vp zone beneath the Himalayan block to the south of the YTS extending to ~150 km depth, which might be associated with the fragmentation of the underthrusting Indian continental lithosphere (ICL) and induce localized upwelling of asthenospheric materials from the upper mantle. In addition, significant low-Vp anomalies were observed beneath the Yadong–Gulu rift and the Cona–Sangri rift extending to ~300 km depth, indicating that the tearing of the subducted ICL might provide pathways for the localized asthenospheric materials upwelling, which contributes to the widespread distribution of north–south trending rifts and geothermal activities in southern Tibet.

Crustal structure of the Tibetan Plateau and adjacent areas revealed from ambient noise tomography

How the Tibetan Plateau deformed in the Indian-Eurasian continental collision has been long debated, more specifically, over the relationship between the deep processes and surface structural complexity. Here, we use ambient noise tomography to obtain a high-resolution crustal S-wave velocity model beneath the Tibetan Plateau and adjacent areas involving a comprehensive dataset from over 500 stations. Our images reveal that the crustal flow should be in a limited scale according to the intermittent low-velocity zones (LVZs) observed in the middle crust at 20–40-km depth of the Tibetan Plateau. The distributions and strengths of LVZs further imply that different deep processes promote the surface deformation in various regions of the Tibetan Plateau. The LVZs in the northern plateau, collocated potassic magmatism and low velocity anomalies in the upper mantle, should be originated from the lithospheric delamination. However, in the southern plateau, the S-wave velocity showed an apparent lateral segmentation feature correlated with the north–south trending rifts. The feature indicates that the LVZs were likely controlled by the lateral tearing of the subducted Indian mantle lithosphere, which promotes the rifting deformation. Moreover, the LVZ in the central Tibet should have contributed to the formation of the conjugate strike-slip fault system. In the Tarim Basin, our model showed a high-velocity anomaly in the lower crust that may be related to ancient mantle plume activity.

Late Cretaceous to Early Cenozoic initiation of rifting of the Windhoek Graben, Namibia

Abstract The Windhoek Graben is a north-south trending rift in central Namibia that forms a prominent topographic feature bisecting an area of plateau uplift. It occupies a potentially crucial role in the propagation of the Late Cenozoic Southwest African Rift system regarding a possible continuation to the west of the Eiseb Rift. It is an unusual example of intra-continental rifting because it has no significant sediment fill associated with the period of active rifting, and hence the timing of rift activity and its tectonic relevance has not hitherto been established. To constrain the age of the Windhoek Graben we examine its regional geomorphic context and its relationship to four sites of igneous activity in the central Namibian Highlands. Two of these consist of clusters of eroded phonolitic tholoid bodies that have yielded 40Ar/39Ar dates of 32 Ma and 52 Ma, respectively, that we use to bracket the age of formation of a prominent remnant land surface, termed here the P52 Surface. From previous mapping of older intrusive igneous bodies, we argue that an even older land surface is partially preserved on the highest features in the area, and this surface (termed PRS) defines an initial domally uplifted surface from which initial drainage radiated, and onto which the earliest volcanic products associated with the Graben were erupted. In particular, the strong similarity in dyke and fault orientations is used to argue for a causal connection between the earliest magmatic activity and the onset of rifting. Long range correlation of PRS into the adjacent Aranos Basin strongly suggests a Late Cretaceous age for this earliest magmatic activity and the onset of rifting, but we cannot exclude a younger origin, any time up to the Early Eocene.

S-wave velocity characteristics and mineralization of the southern tibet detachment system-yalaxiangbo dome

A series of extensional structures, including the southern Tibet detachment system (STDS), the north-south trending rifts (NSTR), and the northern Himalayan gneiss dome (NHGD), developed from the collision and compression between the Indian and Eurasian plates. These tectonic movements were accompanied by magmatism and polymetallic mineralization. Cuona Rift (CR) is located on the STDS next to the Yalaxiangbo Dome (YD) and passes through the Zhegucuo-longzi fault (ZLF), the Lhozhag fault (LZF), the Rongbu-Gudui fault (RGF), the Cuonadong dome (CD), and the YD. The study area contains numerous metal deposits, such as rare metal ore, lead zinc ore, gold deposits, and two geothermal fields, i.e., the Cuona geothermal field (CGF) and the Gudui geothermal field (GGF). Current research on the geological structures from the STDS to the YD is mainly based on magnetotelluric and natural seismic imaging. These surveys have a low resolution, making it impossible to image the shallow crust in detail. This study obtained about a 112 km S-wave velocity profile from the STDS to the YD using the multichannel surface wave imaging method. The profile results indicated that the average thickness of the sedimentary layer from the STDS to the YD is 400–500 m, while it is more than 800 m at certain fault zones. The CD is connected to the high-velocity body below the Zhaxikang ore concentration area (ZOCA) and may have the same provenance. The thermal conductivity reveals that the CGF, the GGF, and the ZOCA have high values and a more intense thermal radiation capacity. This drives the migration and circulation of the thermal fluids in the CGF and the GGF, causing them to continuously transmit heat to the shallow surface along the fault system. The migration of the thermal fluids extracts useful elements from the geological bodies through which it flows. When these elements mix with the atmospheric infiltration water, it precipitates to form the Zhaxikang hydrothermal superimposed transformation type lead-zinc polymetallic deposit.

Origin of the Miocene Adakitic Rocks and Implication for Tectonic Transition in the Himalayan Orogen: Constraints from Kuday Granitoid Porphyry in Southern Tibet

Abstract Postcollisional adakitic magmatism in the Himalayan Orogen provides a probe into the evolution of the collisional orogen. During the Miocene, the Himalayan Orogen underwent a tectonic transition, which was characterized by a series of tectonic events, including the activity of the North-South Trending Rift, exhumation of eclogite, rapid uplift of the orogen, and the extensive adakitic rocks. In this study, we reported the geochemistry and geochronological data of the Kuday dikes intruding into the Tethys Himalayan Sequence near the Sakya Dome of southern Tibet. The Kuday dikes are granitoid porphyries with zircon U-Pb ages of ca. 11 Ma. The Kuday granitoid porphyry dikes have high SiO2 (63.01–68.41 wt.%) and Al2O3 (17.31–19.87 wt.%) but low Mg (0.88–1.41 wt.%), Mg# (36–50), Ni (2.8–19.3 ppm), and Cr (2.9–26.4 ppm), indicating no input of mantle material. They have high Sr (934–1881 ppm), (La/Yb)N (18.84–113.13), and Sr/Y ratios (89.25–305.85) but low K2O/Na2O ratios (0.17–0.79), indicating that they are adakitic affinity. They display initial 87Sr/86Sr ratios of 0.707–0.711 and εNdt values of -3.7–-6.7. These geochemical signatures indicate that the Kuday granitoid porphyrite dikes were derived from the partial melting of the thickened lower crust of the Himalayan Orogen. Partial melting of the thickened lower crust requires additional heat, so the delamination model with lithospheric mantle thinning and asthenospheric upwelling is proposed to explain the formation of the Kuday adakitic rock. The delamination model can also provide a reasonable explanation for the tectonic events during the Miocene in the Himalayan Orogen.

Sedimentary and crustal velocity structure of Trans-North China Orogen from joint inversion of Rayleigh wave phase velocity and ellipticity and some implication for Syn-rift volcanism

We present a high-resolution 3-D shear wave velocity model of the crust and uppermost mantle beneath the Trans-North China Orogen and its adjacent regions derived from a joint inversion of Rayleigh wave phase velocity and ellipticity computed from ambient noise data. The 3-D velocity model reveals large lateral variations in sedimentary and crustal structure across the study area. In addition to the large velocity contrast between the North China Basin and the Ordos Basin located in the east and west sides of the orogen, we found a northward thinning of sediments within the Weihe-Shanxi Rift System that sits in the middle of the orogen, suggesting that the rifting has likely propagated from the Weihe Rift in the south to the Shanxi Rift in the north during the Oligocene-Pliocene. While the north-south trending rift is underlaid by a low Vs across the entire range in the upper and middle crust, only the north segment, which houses the Datong volcanic complex, exhibits a substantial low velocity structure in the lower crust and the uppermost mantle. The center of the low velocity anomaly extends into the uppermost mantle at a location approximately 150 km west to the volcanic complex, which is speculated for a nearly horizontal channel in the lower crust for the upwelling of the hot materials from the asthenosphere. These observations suggest that Shanxi Rift in the north is a more active segment within the rift system, which is manifested by the Quaternary syn-rift Datong volcanic complex.

Miocene Sn polymetallic mineralization in the Tethyan Himalaya, southeastern Tibet: A case study of the Cuonadong deposit

The Cuonadong deposit is located in the Cuonadong gneiss dome of the eastern Tethyan Himalaya and is characterized as a large- to superlarge-scale Sn polymetallic deposit in the Himalaya orogen. The mineralization pattern includes skarn W-Sn mineralization in the strong shear zone of the dome mantle, hydrothermal Sn-W veins controlled by north-south-trending rifts (NSTRs), and Be-Rb mineralization in pegmatitic leucogranite. The results of this study show that the zircon U-Pb age of undeformed (postkinematic) leucogranites is 15.5 ± 0.1 Ma (n = 28, MSWD = 1.4), the cassiterite U-Pb age of the vein-type Sn-W orebody is 14.3 ± 0.5 Ma (n = 40, MSWD = 3.2), and the muscovite Ar-Ar age of beryl-bearing pegmatitic leucogranite is 15.2 ± 0.2 Ma (MSWD = 1.5). The isotopic dating results demonstrate that the Cuonadong Sn-W-Be-Rb mineralization has a close genetic relationship with the Miocene (ca. 16–14 Ma) leucogranite magmatic hydrothermal activity in the Cuonadong dome. The deformed (synkinematic) two-mica granite (18 Ma) was cut by the Sn-W ore veins (14 Ma) faulted by NSTRs, indicating that the E-W-trending extension of the NSTRs initiated at ca. 18–14 Ma. The Himalaya leucogranite belongs to a highly fractionated granite with high Sn content (>15 ppm). In conclusion, the Himalaya leucogranite is considered to be tin granite in this study, and therefore, the Himalaya presents high potential for Sn polymetallic mineralization prospecting.

Reply to the comment titled “The usage of 238U/207Pb vs 206Pb/207Pb linear regressions for the LA-ICP-MS U-Pb dating of cassiterite” on the manuscript “Geological, geochemical, and geochronological characteristics of Weilasituo Sn-polymetal deposit, Inner Mongolia, China”

The Cuonadong deposit is located in the Cuonadong gneiss dome of the eastern Tethyan Himalaya and is characterized as a large- to superlarge-scale Sn polymetallic deposit in the Himalaya orogen. The mineralization pattern includes skarn W-Sn mineralization in the strong shear zone of the dome mantle, hydrothermal Sn-W veins controlled by north-south-trending rifts (NSTRs), and Be-Rb mineralization in pegmatitic leucogranite. The results of this study show that the zircon U-Pb age of undeformed (postkinematic) leucogranites is 15.5 ± 0.1 Ma (n = 28, MSWD = 1.4), the cassiterite U-Pb age of the vein-type Sn-W orebody is 14.3 ± 0.5 Ma (n = 40, MSWD = 3.2), and the muscovite Ar-Ar age of beryl-bearing pegmatitic leucogranite is 15.2 ± 0.2 Ma (MSWD = 1.5). The isotopic dating results demonstrate that the Cuonadong Sn-W-Be-Rb mineralization has a close genetic relationship with the Miocene (ca. 16–14 Ma) leucogranite magmatic hydrothermal activity in the Cuonadong dome. The deformed (synkinematic) two-mica granite (18 Ma) was cut by the Sn-W ore veins (14 Ma) faulted by NSTRs, indicating that the E-W-trending extension of the NSTRs initiated at ca. 18–14 Ma. The Himalaya leucogranite belongs to a highly fractionated granite with high Sn content (>15 ppm). In conclusion, the Himalaya leucogranite is considered to be tin granite in this study, and therefore, the Himalaya presents high potential for Sn polymetallic mineralization prospecting.

Possibilities and problems associated with travertines and tufas in Quaternary studies: A case of the Tibetan Plateau

Travertines and tufas, precipitated from waters (hydrothermal and ambient temperature karstic waters respectively) enriched in calcium and bicarbonate ions, are widespread in continental settings. Both carbonate deposits are important archives for Quaternary studies, as they can be absolutely dated (e.g. via uranium-series dating) and provide evidence of climatic and environmental conditions at the time of formation. Yet, most of the travertines and tufas in China still lack investigation and little is known about the nature of geological and paleoenvironmental information stored in these deposits. Here we summarize previous studies of travertines and tufas from the Tibetan Plateau, with an emphasis on possibilities and problems associated with these carbonates in Quaternary studies, and lastly provide our perspectives on the future of travertine and tufa research. Travertines are quite common on the Tibetan Plateau and preferentially occur along active tectonic zones, such as the approximately north-south trending rift systems and some strike-slip faults caused by the uplift of the Plateau and subsequent east-west extensional deformation of the Tibetan crust during the Late Cenozoic. At present these travertines are under-researched, but several previous studies have shown that the travertine deposition on the Plateau is not only controlled by tectonics but also closely associated with the intensity of the Indian summer monsoon. Some lacustrine tufas deposited on the paleoshorelines of several closed-basin lakes on the Plateau are able to reconstruct lake-level (and hence paleomonsoon) change. It is therefore expected that the widely-distributed travertines and tufas on the Tibetan Plateau have great potential to provide insight into past monsoonal climate change. Furthermore, travertines and tufas preserved in different terrestrial settings, such as periglacial slope, fluvial, and lacustrine environments on the Plateau, appear to be a promising tool for unravelling climate-driven surface processes and geomorphological change of an area. The formation of travertines (e.g. travertine fissure-ridge) is often linked with active tectonics and can thus provide more evidence of faulting and geothermal activities on the Plateau. In several instances, archeological finds (e.g. ancient human hand- and footprints) are stratigraphically or spatially associated with travertines or tufas; thus the dating of these carbonates may provide the timing of human migration and settlement of the Tibetan Plateau, and allow us to better understand the prevailing climatic and environmental conditions during site occupation and abandonment. Several problems might be encountered in the study of the travertines and tufas. Firstly, the travertines and tufas tend to accumulate discontinuously due to the tempo-spatial shift in spring orifice location and water flow pattern, and may be mixed with terrigenous sediments. Another problem is that a variety of travertine and tufa lithofacies and fabrics within these deposits are observed, and thus a detailed petrographic analysis is needed. In addition, these carbonate deposits are prone to early diagenetic alteration as they commonly have relatively high initial porosity and permeability, resulting in modification of the original geochemical composition, and hence, the paleoclimatic proxy signals recorded in these deposits. Thus, careful analysis must be conducted in order to distinguish primary fabrics from altered material. Last but not least, the processes responsible for travertine and tufa precipitation are extremely complex, and often lead to difficulties in the interpretation of climatic and environmental signals stored in these deposits. Further research should focus on determining the relationships between travertine and tufa proxies (e.g. oxygen and carbon stable isotopes) and climate (temperature, precipitation) based on field measurements and experiments carried out in different depositional environments, such as proximal vents, intermediate slopes, or distal flats and depressions. Considering the problems mentioned above, we suggest that a multidisciplinary approach, including geomorphological, sedimentological, petrographic, geochronological and geochemical investigations, is necessary in the study of travertines and tufas.

First field identification of the Cuonadong dome in southern Tibet: implications for EW extension of the North Himalayan gneiss dome

The Cuonadong dome exposes in east-southern margin of the North Himalayan gneiss domes (NHGD), which is reported first time in this study. The Cuonadong dome is located at the southern part of the Zhaxikang ore concentration area, which is divided into three tectono-lithostratigraphic units by two curved faults around the dome geometry from upper to lower (or from outer to inner): the upper unit, middle unit and lower unit, and the outer fault is Nading fault, while the inner fault is Jisong fault. The Cuonadong dome is a magmatic orthogneiss and leucogranite mantled by orthogneiss and metasedimentary rocks, which in turn are overlain by Jurassic metasedimentary and sedimentary rocks. The grades of metamorphism and structural deformation increase towards the core, which is correspondence with the Ridang Formation low-metamorphic schist, tourmaline granitic–biotite gneiss, garnet–mica gneiss and mylonitic quartz–mica gneiss. The Cuonadong dome preserves evidences for four major deformational events: firstly top-to-S thrust (D1), early approximately N–S extensional deformation (D2), main approximately E–W extensional deformation (D3), and late collapse structural deformation (D4) around the core of the Cuonadong dome, which are consistent to three groups lineation: approximately N–S-trending lineation including L1 and L2, E–W trending L3, and L4 with plunging towards outside of the dome, respectively. The formation of the Cuonadong dome was probably resulted from the main E–W extensional deformation which is a result of eastward flow of middle or lower crust from beneath Tibet accommodated by northward oblique underthrusting of Indian crust beneath Tibet. The establishment of the Cuonadong dome enhanced the E–W extension of the NHGD, which is further divided into two structural dome zones according to the different extensional directions: approximately N–S extensional North Himalayan gneiss domes (NS-NHGD) and E–W extensional North Himalayan gneiss domes (EW-NHGD). The NS-NHGD developed by a dominantly N–S contraction and locally extensional regime and keep a close relationship to the South Tibetan Detachment System, whereas the EW-NHGD formed by an E–W extensional deformation along the north–south-trending rifts.

Stream profile analysis, tectonic geomorphology and neotectonic activity of the Damxung‐Yangbajain rift in the south Tibetan Plateau

AbstractThe Damxung‐Yangbajain rift is one of the most active north–south trending rifts in the south Tibetan Plateau, and it has been playing an important role in accommodating the east–west extension of the Tibetan Plateau. Both stream profiles on the Nyainqentanglha Range adjacent to the northwest part of the Damxung‐Yangbajain rift and tectonic geomorphology in the north of the rift are analyzed to assess the spatial pattern and intensity of rock uplift which is related to neotectonic activity. A total of 85 stream profiles across the Nyainqentanglha Range are analyzed, and 111 knickpoints are interpreted. Most of these stream profiles are characterized by prominent convexities with two or more knickpoints, many of which are formed due to the strong rock uplift evidenced by abnormal concavity and extremely high steepness indices during the Quaternary. Neotectonic activity in this region is well replicated in the stream profile indices and offset landforms. Tectono‐geomorphic analysis shows that the concavity and steepness indices correlate with the fault movements at many places. The Damxung‐Yangbajain rift is characterized by left‐lateral strike‐slip in the north of Damxung and by normal movement in middle and southern parts. The middle and southern parts have been undergoing higher uplift than has the northern area. It is most likely that the strong uplift is related to the heat flow under the crust. Earthquakes occurring in the Damxung‐Yangbajain rift, including a M8 in 1411 and M6.6 in 2008, are thought to be related to heat flow activity. All of the stream profile indices and tectonic geomorphology show that the Damxung‐Yangbajain rift is not in a stable state. Copyright © 2016 John Wiley & Sons, Ltd.

Post-Caledonian extension in the West Norway–northern North Sea region: the role of structural inheritance

Abstract The northern North Sea region has experienced repeated phases of post-Caledonian extension, starting with extensional reactivation of the low-angle basal Caledonian thrust zone, then the formation of Devonian extensional shear zones with 10–100 km-scale displacements, followed by brittle reactivation and the creation of a plethora of extensional faults. The North Sea Rift-related approximately east–west extension created a new set of rift-parallel faults that cut across less favourably orientated pre-rift structures. Nevertheless, fault rock dating shows that onshore faults and shear zones of different orientations were active throughout the history of rifting. Several of the reactivated major Devonian extensional structures can be extrapolated offshore into the rift, where they appear as bands of dipping reflectors. They coincide with large-scale boundaries separating 50–100 km-wide rift domains of internally uniform fault patterns. Major north–south-trending rift faults, such as the Øygarden Fault System, bend or terminate against these boundaries, clearly influenced by their presence during rifting. Hence, the North Sea is one of several examples where pre-rift basement structures oblique to the rift extension direction can significantly influence rift architecture, even if most of the rift faults are newly-formed structures.

Knickpoint evolution on the Yarlung river: Evidence for late Cenozoic uplift of the southeastern Tibetan plateau margin

A salient geomorphic feature of the Yarlung River is its abundance of large knickpoints, which in many cases coincide with north–south trending rifts. Across one of these rifts, near the town of Jiacha, the Yarlung falls nearly 500 m from an elevation of ∼3500 m over 80 river kilometers, making this the second largest knickpoint on the river. We propose that the Jiacha knickpoint represents a wave of incision migrating upstream through the drainage network in response to a downstream base level fall, not a disturbance in the channel to due rift tectonics.Longitudinal profile slope-area and chi (χ) analysis of Yarlung River tributaries and those of several major rivers in southeastern Tibet indicate several knickpoints are present at ∼3500 m elevation, all resulting from a single regional-scale base level fall. Retreat rates calculated from celerity modeling indicate that the Jiacha knickpoint was located at the upstream edge of the Namche Barwa massif at ∼10 Ma, a history consistent with apatite 4He/3He thermochronometry data and thermokinematic modeling from that region. These data suggest the Yarlung River has flowed in its present course through this area since at least 10 Ma and imply that at least 500 m of incision occurred within this canyon over this time period. The spatial scale of these observations suggests that these knickpoints resulted from surface uplift of southeastern Tibet of 500 to 2500 m just prior to ∼10 Ma. Additionally, our mapped knickpoint locations indicate that reorganization of the drainage network just east of the Namche Barwa massif occurred prior to this time.

Distribution of the low velocity bulk in the middle-to-lower crust of southern Tibet: implications for formation of the north–south trending rift zones

We conducted the ambient noise tomography to image the shallow crustal structure of southern Tibet. The 2D maps of phase velocity anomalies at the periods of 10–16 s show that the low velocities are mainly confined along or near some of the rift zones. While the maps at the periods of 18–25 s show that the coherent patterns that the low velocities expand outside of the rift zones. It means that the low velocities are prevailing in the middle crust of southern Tibet. According to the previous study of surface wave tomography with teleseismic data, we find that the low velocities from the lower crust to the lithospheric mantle are also restricted to the same rift zones. Thus, the integrated knowledge of the distribution of the low velocities in southern Tibet provides some new insight on the formation of the north–south trending rift zones. Compiling the multidiscipline evidences, we conclude that the rifting was an integrated process of the entire lithosphere in the early stage (~26–10 Ma), but mainly occurred within the upper crust due to the weakening a decoupling in the low velocity middle crust in the late stage (later than ~8 Ma).

Middle-Miocene transformation of tectonic regime in the Himalayan orogen

Understanding the multiple tectonic transformations during the Himalayan orogeny is significant in evaluating the evolution of Himalayan orogen. In the Gyirong area in south Tibet, deformed leucogranitic veins in the biotite-plagioclase gneisses of Greater Himalayan crystalline complex (GHC) constitute south-vergent asymmetric folds. The reconstruction of the veins shows that they experienced two generations of deformation under different tectonic regimes: an earlier top-to-north extension and a later top-to-south thrusting, implying a tectonic transformation from N-S extension to N-S shortening. Zircons LA-ICP-MS U-Pb dating of the leucogranite shows that it was emplaced during 21.03-18.7 Ma. The data suggest that the tectonic transformation occurred after 18.7 Ma. The chronological data of South Tibet detachment system (STDS) and North-South trending rift (NSTR) from Gyirong and other areas indicate that the Himalayan orogeny was in a period of tectonic transformation from N-S extension to N-S shortening during 19-13 Ma. The transformation of tectonic regime was probably controlled by the India-Asia convergence rate. An increase in the convergence rate resulted in N-S shortening of the orogen, thrusting and folding, with coeval formation of the NSTR in Tibet. A decrease in the convergence rate led to N-S extension and reactivation of the STDS.

Seismic P-wave tomography in eastern Tibet: Formation of the rifts

For better studying the relationship between the rifts and deep structure, a detailed P-wave velocity structure under eastern Tibet has been modeled using 4767 arrival times from 169 teleseismic events recorded by 51 portable stations. In horizontal slices through the model, a prominent low-velocity anomaly was detected under the rifts from the surface to a depth of ∼250 km; this extends to a depth of ∼400 km in the vertical slice. This low-velocity anomaly is interpreted as an upper mantle upwelling. The observations made provide seismic evidence for the formation of north-south trending rifts. East of the low-velocity anomaly, a clear high-velocity anomaly is found between depths of 40 and 200 km. Due to its shallow depth, we suggest that it consists of materials from an ancient continental closure rather than the Indian Plate. From depths of 250 to 400 km, a high-velocity anomaly appears to the south of the Jiali Fault. This anomaly may correspond to the northern edge of the Indian Plate that detached from the surface under the Himalayan block. We suggest that the Indian Plate underthrusts no further than the Jiali Fault in eastern Tibet.

3-D lithospheric structure beneath southern Tibet from Rayleigh-wave tomography with a 2-D seismic array

SUMMARY A 3-D shear wave velocity model has been established for the lithosphere of southern Tibet through Rayleigh-wave tomography. The teleseismic data are acquired from a 2-D seismic array selected out of the second phase of the Hi-CLIMB project. We first inverted for a 2-D phase velocity model using two-plane-wave tomography approach, and then inverted for the 3-D shear wave velocity model with the 2-D phase velocity model and the map of crustal thickness derived from receiver functions. The results reveal a pervasive low velocity anomaly in the middle crust of the Lhasa block. We interpret this low velocity anomaly as the presence of wholesale mid-crustal channel flow. Prominent low velocity anomalies from the lower crust to the mantle lithosphere are observed along the north–south trending rifts: the Tangra Yum Co rift and the Pumqu Xianza rift. We propose a possible scenario for the formation of these rifts: the entire lithosphere is involved in the early-stage rifting, while the late-stage rifting is restricted in the upper crust. In the non-rift regions, a slab-like high velocity anomaly is traced beneath both the Himalayas and Lhasa block. Combining all the observations, we suggest that the central part of southern Tibet is currently underlain by the broken Indian lower crust and mantle lithosphere.

Crustal and mantle velocity models of southern Tibet from finite frequency tomography

[1] Using traveltimes of teleseismic body waves recorded by several temporary local seismic arrays, we carried out finite-frequency tomographic inversions to image the three-dimensional velocity structure beneath southern Tibet to examine the roles of the upper mantle in the formation of the Tibetan Plateau. The results reveal a region of relatively high P and S wave velocity anomalies extending from the uppermost mantle to at least 200 km depth beneath the Higher Himalaya. We interpret this high-velocity anomaly as the underthrusting Indian mantle lithosphere. There is a strong low P and S wave velocity anomaly that extends from the lower crust to at least 200 km depth beneath the Yadong-Gulu rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. Intermediate-depth earthquakes in southern Tibet are located at the top of an anomalous feature in the mantle with a low Vp, a high Vs, and a low Vp/Vs ratio. One possible explanation for this unusual velocity anomaly is the ongoing granulite-eclogite transformation. Together with the compressional stress from the collision, eclogitization and the associated negative buoyancy force offer a plausible mechanism that causes the subduction of the Indian mantle lithosphere beneath the Higher Himalaya. Our tomographic model and the observation of north-dipping lineations in the upper mantle suggest that the Indian mantle lithosphere has been broken laterally in the direction perpendicular to the convergence beneath the north-south trending rifts and subducted in a progressive, piecewise and subparallel fashion with the current one beneath the Higher Himalaya.